Turbochargers are a type of forced induction system. They deliver air to an engine intake at greater density than would be possible in a normally aspirated configuration. This allows more fuel to be combusted, thus boosting an engine's horsepower without significantly increasing engine weight. A smaller turbocharged engine can replace a normally aspirated engine of a larger physical size, thus reducing the mass and aerodynamic frontal area of a vehicle.
Generally, turbochargers use exhaust flow from an engine exhaust manifold, which enters a turbine housing at a turbine inlet, to drive a turbine wheel, which is located in the turbine casing or housing. The turbine wheel is solidly affixed to one end of a shaft, and a compressor wheel is mounted to the other end of the shaft wherein the turbine wheel provides rotational power to drive the compressor wheel. Once the exhaust gas has passed through the turbine wheel and the turbine wheel has extracted energy from the exhaust gas, the spent exhaust gas exits a turbine exducer of the turbine housing and is ducted to a vehicle downpipe and usually to after-treatment devices such as catalytic converters, particulate traps and NOx traps. The power developed by the turbine stage is a function of the expansion ratio across the turbine stage, i.e., the expansion ratio from the turbine inlet to the turbine exducer. The range of the turbine power is a function of, among other parameters, the mass flow through the turbine stage.
The compressor stage consists of the compressor wheel and its housing. Filtered air is drawn axially into an inlet of a compressor cover by the rotation of the compressor wheel. The power that is generated by the turbine stage through the turbine wheel is transferred through the shaft to drive the compressor wheel and produce a combination of static pressure with some residual kinetic energy and heat. The pressurized gas exits the compressor cover through a compressor discharge and is delivered, usually via an intercooler, to the engine intake.
In designing the turbine stage, selection of the turbine stage components is made relative to a preferred performance point. Reciprocating internal combustion engines have long been equipped with such turbochargers. In a simple uncontrolled fixed-nozzle turbocharger system, the maximum charging pressure is a function of the strength of the engine. The uncontrolled turbocharger must thus be so designed that the optimal performance is reached only at high engine speeds. However, at other speed regions the turbocharger provides suboptimal boost or air volume to the engine.
Controlled turbochargers provide improved performance, in that the turbine optimal operating point can be reached at low or medium engine speeds. In a simple controlled system, when the flow rate of exhaust gases increases and the turbocharging pressure becomes too high, part of the exhaust gases are simply discharged into the surrounding atmosphere through a wastegate so as to bypass the turbine, whereby damage to the engine due to excessive boost at high speeds can be avoided. However, since exhaust gases bypass the turbine through the wastegate, energy losses are higher and the engine performance drops at high speeds.
It is also known to provide multiple flow conduits within a single turbine casing or housing, such that the exhaust gas flow through the single turbine casing and the turbine performance can be controlled to perform alternately as a low pressure and a high pressure turbine. In one example, these casings can be classified as a twin scroll or twin-flow casing.
In a twin-flow casing the spiral turbine casing is divided by at least one radial partition into two axially adjacent spirals. The exhaust gas of each spiral enters the turbine wheel inlet to impact the periphery of the turbine wheel, with axially adjacent spiral conduits impacting axially adjacent segments of the turbine wheel.
The selection or operation of the spirals can be controlled via a gate valve (throttle valve, flap, slide valve) which enlarges flow cross section with increasing turbocharger speed. A control device is generally provided with sensing means for sensing boost pressure or speed, and an adjustment member for actuating the gate valve.
As one example of a turbocharger, U.S. Pat. No. 6,652,224 (Mulloy et al.) discloses a variable geometry turbine which includes a radial turbine wheel with movable nozzle vanes controlling flow from a single volute. Similarly, U.S. Pat. No. 6,742,986 (Osako et al.) discloses a turbocharger with a radial turbine, which is formed as a variable displacement turbine to vary the turbine capacity. These designs have increased complexity.
Alternatively, a twin scroll design may be used. U.S. Pat. No. 3,614,259 (Neff) teaches a divided turbine casing which may be used to provide either a pulse turbine or a variable speed turbine, with gas flow controlled via a flapper valve. In the case of an impulse turbine, multiple exhaust gas lines are coupled to the turbine casing, such that the gate valve or flow control means must be of commensurate complexity, controlling flow through two or four or more flow paths.
U.S. Pat. No. 4,544,326 (Nishiguchi et al.) discloses a variable-capacity turbine which defines first and second scroll passages in the turbine scroll passage which supplies exhaust gas to a radial turbine. A rotating valve is provided to selectively open and close the second scroll passage depending upon pressure increases and decreases in exhaust gas flow. Alternatively, U.S. Pat. No. 7,363,761 (Dickerson) discloses a turbocharger with an exhaust gas throttle to control the flow of exhaust to the turbine.
U.S. Pat. No. 5,092,126 (Yano) discloses a twin scroll turbine which uses a radial turbine, a twin scroll structure and a variable nozzle structure. In a first embodiment, a first scroll passage provides a fixed flow capacity, while a second scroll passage provides variable flow through a variable area nozzle unit comprising fixed and movable vanes which control variable flow into the radial turbine from the second scroll passage. In a second embodiment, a control is provided in a first scroll passage so that it is possible to adjust a variable area nozzle unit with the control valve kept closed and then to open up both the variable area nozzle unit and the control valve.
U.S. Pat. No. 6,983,596 (Frankenstein et al.) discloses a controlled turbocharger having a twin-flow design with an integrated bypass using a rotary valve.
In another design shown in FIG. 1, a twin scroll design may be provided with a first valve and actuator assembly which controls a wastegate while a second valve and actuator assembly controls the flow of exhaust gas into a second scroll which can be selectively opened and closed. However, the provision of two valve and actuator assemblies increases the overall cost and complexity of the turbocharger and can result in inefficiencies in turbocharger operation due to the use of the wastegate structure.
It is an object of the present invention to therefore overcome disadvantages associated with these turbochargers.